screenshare: Add XDP portal ScreenCast support

With Wayland we can't just share X11 windows/screens; we need to ask for
a stream from the compositor. Add support for that, falling back to the
direct mode if it isn't present (since *Pidgin* can be using X11 even
when the system is Wayland).

https://flatpak.github.io/xdg-desktop-portal/portal-docs.html